Failure: (ID: 838926df) did not find any jar files with a Main-Class manifest entry

There can be one or many issues because of which you get this error. For resolution please check below things -

1. Your app.yaml should have entrypoint and runtime information as below -

   runtime: java11
entrypoint: java -Xmx64m -jar blahblah.jar

2. your pom.xml should have appengine maven plugin dependency

`

<plugin>
   <groupId>com.google.cloud.tools</groupId>    
   <artifactId>appengine-maven-plugin</artifactId>
   <version>2.2.0</version>
</plugin>

`

3. don't modify jar if you want to replace any configs use command -
   jar uf blahblah.jar filename.yaml

4. make sure you have packaging as a jar in pom.xml like this - <packaging>jar</packaging>

I deleted my maven plugin by accident, so don't delete it.

      <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
      </plugin>

Try to comment out in pom.xml packaging into war file, example:

<!--  <packaging>war</packaging> -->

It fixed the issue for me. Good minimal+workable example is here: https://codelabs.developers.google.com/codelabs/cloud-app-engine-springboot#0

I was able to fix it for my AppEngine + Gradle project by adding

jar {
    enabled = false
    archiveClassifier = ''
}

to the build.gradle file.